Fig. 3.

L-AP4 decreases the efficiency with which spikes encode contrast. A: the response to a dark flash was a depolarization and a burst of spikes. Membrane potential (black trace) was interpolated linearly through spikes. The peak membrane potential (V_p) and average spike rate S were measured in a 40-ms window (gray rectangle). The membrane potential V_m was measured between depolarizations. B: derivation of spike threshold V_th. Graph of spike rate S against peak depolarization V_p, fitted with Eq. 5, where V_th was a free parameter of the fit that defined a threshold potential for spiking (arrow). C: L-AP4 raised noise fluctuations and small depolarizations above spike threshold (V_th, dashed line), triggering spontaneous and evoked spikes (gray and black vertical lines, respectively). D: L-AP4 brought the quantify (V_p − V_th) above zero; thus the V_p evoked by the dark flash V_p was raised above spike threshold V_th. E: L-AP4 increased accuracy at low contrast and decreased accuracy at high contrast. Allowing the cell to recover from L-AP4 and then depolarizing it to the membrane potential measured in L-AP4 increased accuracy at low contrast. F: efficiency with which spikes detected contrast was expressed in units (contrast·spikes/s)⁻¹. The efficiency was calculated from contrasts at two criterion accuracies (90% and 68%; see text). For either criterion contrast, efficiency was reduced by L-AP4. Asterisks (*) show values statistically different from zero by t-test. G: efficiency is plotted against the membrane potential V_m referenced to the spike threshold V_th, expressed in units V_m − V_th. As V_m was altered by injecting current, efficiency peaked at the resting potential of the cell (gray rectangle indicates mean resting potential ± SE).